1. Field of the Invention
The present invention relates to a method for manufacturing an SOI substrate having a semiconductor layer on its insulating surface.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) using a semiconductor thin film (a thickness of approximately several nanometers to several hundred nanometers) formed over a substrate having an insulating surface of glass or the like has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as integrated circuits and electro-optic devices, and rapid development of thin film transistors that are to be used as switching elements in image display devices, in particular, has been pushed.
Instead of silicon wafers that are manufactured by thinly slicing an ingot of a single crystal semiconductor, semiconductor substrates called silicon on insulator (SOI) substrates have been developed, which have a thin single crystal semiconductor layer over an insulating layer, and are more widely used as substrates in manufacturing microprocessors or the like. An integrated circuit using an SOI substrate draws attention as an integrated circuit in which parasitic capacitance between drains of transistors and a substrate can be reduced, performance of the semiconductor integrated circuit can be improved, and low power consumption is achieved.
As a method for manufacturing SOI substrates, a hydrogen ion implantation separation method is known (for example, see Reference 1: Japanese Published Patent Application No. 2004-134675). For example, ions of hydrogen or the like are implanted into a silicon wafer of which surface is provided with a silicon oxide film to form a damage region at a predetermined depth from the surface, and a thin film single crystal silicon layer (SOI layer) separated in the damage region is bonded to another wafer. Further, in the technique the bonding strength of the SOI layer is increased by heat treatment to form an SOI substrate. Note that the hydrogen ion implantation separation method may also be referred to as a Smart Cut (registered trademark) method.
On the other hand, forming of an SOI layer on an insulating substrate of glass or the like is also attempted. As an example of an SOI substrate in which an SOI layer is formed on a glass substrate, an SOI substrate in which a thin single crystal silicon layer (SOI layer) is formed on a glass substrate having a coating film by a hydrogen ion implantation separation method is known (for example, see Reference 1: Japanese Published Patent Application No. 2004-134675). In this case also, hydrogen ions are introduced into a silicon wafer to form a damage region at a predetermined depth from the surface, and the silicon wafer is separated in the damage region after bonding the glass substrate and the silicon wafer together; thus, a thin single crystal silicon layer is formed over a glass substrate.